The present invention relates to a digging apparatus with the aid of which pipelines and cables can be placed in a trench in the sea bottom after the pipeline or cable has already been placed on the seabottom. This trenching is necessary to prevent damage, such as that caused by the trawler boards of fishing boats, which are dragged along the sea bottom.
When selecting the digging method one should first note a difference between burying in a sea bottom consisting substantially of sand, and in a sea bottom consisting substantially of clay. Furthermore the depth at which the sea bottom is located plays an important part in the selection of the digging method. As trenching takes place along very large distances, it is possible that the composition of the sea bottom will vary and may consist of clay and of sand.
Prior to the present invention, in shallow water, where the sea bottom generally consisted of sand, a digging method was used, in which water jets and an air displacement caused by pressurized air supplied by tubes from a working ship are used, and in which the sand torn from the bottom was thus discharged. Such a digging method may be implemented with a horse-shoe like apparatus which includes a sledge, which is displaceable along the sea bottom, and which apparatus is placed over the pipeline or cable to be buried. At both sides of the pipeline jet pipes extend into the sea bottom and flush the sand loose. As a consequence there is a resulting air displacement ("air lift") and the torn sand torn away from under the pipeline is discharged. A trench is thus created as the sledge is displaced. The pipeline is placed in the trench after some length of trench is formed, the length being dependent on the flexibility of the pipe. After a caving in of the slope of the dug trench the pipeline is buried to some extent and by the gradual silting as a consequence over the tide flows the covering of the pipeline is gradually increased in depth. As a consequence of the rather small coherence of the sand bottom the caving in of the slope of the trench may have take place already before the pipeline is on the trench bottom, so that the pipeline will not be deep enough in the trench, or is locally uncovered. Furthermore the pipeline may break because it is curved, and locally either not supported or poorly supported.
In water which is deeper than 100 meters it is different to supply pressurized air and water by means of tubes from the working ship and therefore electricity is supplied to the digging machine to drive the underwater pumps placed thereon. In this case, a method of burying the pipeline by means of fluidizing, during which no soil displacement takes place, is preferred, because such method does not require the supply of pressurized air.
When digging a trench in a clay bottom the above mentioned method is not suitable, as for this method considerably larger capacities are required. A significant obstacle is the problem of removing the soil directly under the pipeline, caused by the high coherence of the clay.